The starting point for the present invention is the fact that the flow conditions in pipelines of industrial facilities in all cases are far from ideal, rather they are more or less highly disturbed. Disturbances in the flow conditions are generally in the form of asymmetries of the axial flow velocities of the medium over the cross-section of a pipeline, eddies, i.e. radial velocity components of the medium, and pressure waves within the medium. The asymmetries are caused, for example, by half-closed valves or other components in the cross-section of the pipeline, the eddies arising essentially as a result of bends, in particular sequential bends in different planes, but also on T-junctions within the pipeline system. Pressure waves within the medium are triggered above all by pumps, and also, for example, by changes in the degrees of opening of valves within the adjacent pipelines.
The simplest methods for ultrasound flow rate measurement operate only by means of a pair of ultrasound transducers located on the measuring tube, forming a measuring path. In the case of this ultrasound flow rate measurement method, the measured value for the rate of flow of the medium is determined from the differences in the propagation time of the ultrasound signals along the measuring path upstream and downstream. The measured value for the rate of flow in this case is determined via a constant correction factor, or via a correction factor dependent on the velocity of the medium along the measuring path, from the velocity of the medium along the measuring path. In the case of an ultrasound flow measuring method of this kind, it is not possible to take account of the above-mentioned disturbances of the flow conditions.
The published application PCT/EP 96/05082 discloses an ultrasound flow rate measurement method which compensates for the disturbance of the flow conditions by means of asymmetries of the axial velocities of the medium over the cross-section of the measuring tube essentially by mounting at least two pairs of ultrasound transducers forming a measuring path on the measuring tube, by determining the Reynolds' number along the measuring path from the values for the velocity of the medium along the measuring path, and by correcting the measured value for the rate of flow by means of the value for the Reynolds' number. However, this method also allows the measuring accuracy to be impaired by eddies within the medium.
However, the degree of influence of eddies within the medium represents a very considerable problem with respect to the measurement accuracy of the ultrasound flow measuring method. The disturbances of the flow conditions caused by eddies are particularly troublesome because, on the one hand, eddies present also influence the flow profile of the medium in the axial direction and, on the other hand, die out very slowly. These above-mentioned properties of the eddies are shown particularly clearly from the experimentally determined measurement curves in FIG. 1 of the drawing. FIG. 1 shows the flow conditions of flowing media with Reynolds' numbers of 50,000 and 300,000 in relation to the length of the inlet section, with respect to the diameter with the pipeline. Here, U designates the axial velocity of the medium, while W designates the tangential velocity of the medium, depending on the distance from the measuring tube axis. It is also seen clearly that the eddies have not died down completely after an inlet section with a length corresponding to around 80 times the diameter of the pipeline. Depending on the respective conditions, eddies can also clearly still impair the measurement accuracy even after an inlet section with a length which corresponds to 100 times and more of the pipeline diameter (cf. also the publication "Turbulent Pipe Flow with Swirl", Wendelt Steenbergen, Eindhoven, Eindhoven University of Technology, 1995).
In order to reduce the influence of eddies on the measurement of the rate of flow of a medium, among other things, it has been proposed that a flow equalizer be inserted into the pipeline. However, a flow equalizer of this kind entails problems in that, on the one hand, it causes a clear pressure loss, and, on the other hand, it does not completely eliminate the eddies. Also, flow equalizers cannot be used in connection with flowing media which have a certain portion of solids, since in such a case the flow equalizer would be obstructed within a short time.
EP 0 639 776 A1, on which the invention is based, concerns an ultrasound flow measuring method in the case of which the influence of eddies within the medium on the measured value for the rate of flow is reduced by the fact that the velocities of the medium are measured along two measuring paths with different sensitivities with respect to the eddies. The different sensitivities are provided by the fact that radial components of the measuring paths deviate from one another with respect to the measuring tube axis. In this case the measuring paths pass in such a way that the velocities along the measuring paths are identical when there are no eddies present within the measuring tube, while the difference of the velocities along the measuring paths does not disappear when the flow includes eddies. In the case of the known method, the measured value for the rate of flow of the medium through the measuring tube is corrected by means of this difference in the velocities. The known method is problematic in two regards. On the one hand the difference of the velocities of the medium along the two measuring paths with different sensitivities to eddies has a very considerable cross-sensitivity to disturbances as a result of pressure waves within the medium, as was determined experimentally. Here also the increase in the measurement frequency proposed in EP 0 639 776 A1 does not solve the problem. Since disturbances as a result of pressure waves of course require a different correction of the measured value for the rate of flow than disturbances for eddies, the measured value for the rate of flow cannot be corrected satisfactorily by means of the difference determined in the prior art, if, as is regularly the case, pressure waves impair the flow conditions. On the other hand, eddies, as shown in FIG. 1, are regularly asymmetrical. Such an asymmetry is not taken into account in the case of the known method for ultrasound flow rate measurement.